In a conventional fuel injector, shown in FIG. 1 in longitudinal section, a fuel filter 3 is held by a press fit on a metallic valve housing 1 and a metallic valve body 2 which axially projects from valve housing 1. Fuel filter 3 has a cup-shaped base element 4 which is made of plastic and has four cup sections, 4a, 4b, 4c, 4d, whose inner diameters are graduated with respect to each other. Upper cylindrical cup section 4a, having the largest inner diameter, is slipped over valve housing 1 with a press fit. Second cylindrical cup section 4b, contiguous thereto and having a reduced inner diameter, surrounds the upper region of valve body 2 with a radial clearance and has a wall opening 5 into which a filter mesh 6 is inserted in each case. Third cup section 4c, contiguous to second cup section 4b, tapers conically and, at its lower end, transitions into cylindrical fourth cup section 4d which sits on valve body 2 with a press fit. In the region of conical third cup section 4c, radial inflow ducts 7 are formed in valve body 2, which discharge into a central valve chamber 8 on one side and at the outer wall of valve body 2 on the other side, where they form an intake opening 9 for the fuel flow to valve chamber 8. The other components of the fuel injector such as valve needle 10 which, together with a valve seat 11, releases or seals a spray-discharge orifice 12 in valve chamber 8, solenoid 13 to actuate valve needle 10, and valve-closure spring 14 are well known, for example from Bosch Kraftfahrtechnisches Taschenbuch, [Automotive Handbook] 23rd edition, 1999, pages 473 and 476, so that there is no need to discuss them further.
In order to achieve appropriate clamping between the valve housing and valve body on one side and the plastic filter on the other side, and also reliable mounting, close tolerances of the parts to be interconnected by compression must be observed, and the plastic part must be subjected to special conditioning. If the compression is too high, the plastic base element of the fuel filter may be damaged or destroyed during the pressing-on operation. If the compression force is too low, the filter may detach easily since the base element made of plastic has a different thermal expansion coefficient than the metal of valve housing and valve body. Swelling of the plastic may also cause expansion of the base element so that the water content of the plastic must be adjusted to a specific value by conditioning the plastic base element.